Hydraulic system

ABSTRACT

A hydraulic system comprising an actuator including a housing and a piston movable in the housing, a reversible, variable speed pump, and conduits for connecting the pump to both sides of the piston so that the pump can pump hydraulic fluid from one side of the piston to the other. The pump is driven by a reversible, variable speed drive with the direction of piston movement being a function of the direction in which the pump is driven. The rate of piston movement is a function of the speed at which the pump is driven. The actuator may be an unbalanced actuator, in which event the unbalance can be compensated for by the pump or by suitable piping connections.

nited States Patent [1 1 ilkerson et a1.

[ HYDRAULIC SYSTEM [75] lnventors: William D. Wilkerson, Santa Ana; A.

P. Henry, Irvine, both of Calif.

[73] Assignee: Bertea Corporation, Irvine, Calif.

[22] Filed: Oct. 22, 1971 211 Appl. No.: 191,912

[52] U.S. Cl. 60/476, 91/420 [51] int. Cl. FlSb /18 [58] Field of Search60/52 R, 476-, 91/420 [56] ReterencesCited UNITED STATES PATENTS1,812,587 6/1931 Ellis 60/52 R 2,716,995 9/1955 Baugh et al. 60/52 R2,768,500 10/1956 Tyler 60/52 R July 24, 1973 3,234,957 2/1966 Allen150/52 R X Primary Examiner-Edgar W. Geoghegan Attorney-Gordon L.Peterson et al.

- the other. The pump is driven by a reversible, variable speed drivewith the direction of piston movement being a function of the directionin which the pump is driven. The rate of piston movement is a functionof the speed at which the pump is driven. The actuator may be anunbalanced actuator, in which event the unbalance can be compensated forby the pump or by suitable piping connections.

14 Claims, 3 Drawing Figures /11 Moron CONTROLLER I5,

75 4; MOTOR I f 7t 90 1 Z6 51, 31: w

HYDRAULIC SYSTEM BACKGROUND OF THE INVENTION Hydraulic systems are usedto perform many different control functions. One type of hydraulicsystem ineludes an actuator and a pump. The actuator includes a cylinderand a piston, and the pump selectively pumps hydraulic fluid from oneside of the piston to the other to thereby control movement of thepiston.

One problem with systems of this kind is how to provide fine control ofpiston movement. With prior art devices of this type, it is difficult orimpossible to very accurately position the piston.

The problem of accurate piston positioning is compounded when anunbalanced actuator is used. In an unbalanced or differential actuatormovement of the piston causes a greater change in volume on one side ofthe piston than on the other. It then becomes necessary to provide meansto compensate for the unbalance. Unbalance is typically caused by thepresence of a connecting rod on one side of the piston which effectivelyreduces the volume of the chamber on that side of the piston.

SUMMARY OF THE INVENTION The present invention relates generally to ahydraulic system in which a pump moves hydraulic fluid from one side ofa pressure-responsive member such as an actuator piston to another tothereby control the position of the pressure-responsive member. Thepresent invention provides several improvements to this kind of systemincluding solutions to the abovedescribed problems.

With the present invention, fine control of piston movement is obtainedby using a variable speed drive for the pump. This can be used toadvantage whether the actuator is balance or unbalanced.

The rate of piston movement is a function of the speed at which the pumpis driven. Accordingly, for fine control of piston movement, the pump isdriven at a relatively low velocity. This causes pump delivery rate tobe relatively low and consequently the piston moves slowly.

Although other drive means could be used for the pump, it is preferredto use a variable speed reversible electric motor. The direction ofpiston movement is a function of the direction in which the pump isdriven.

The present invention also provides two novel ways of compensating foractuator unbalance. According to the first of these, the pump includesmeans for compensating for the unbalance. More particularly, the pump isof the barrel-type and includes a valve plate having first, second andthird ports. The piston divides the cylinder into first and secondchambers with the piston being movable to displace more fluid in thefirst chamber than is taken into the second chamber. First and secondconduits provide communication between the first and second ports andthe first and second chambers, respectively. A third conduit providescommunication between the third port and a source of hydraulic fluid.

When the pump is driven in a first direction, hydraulic fluid iswithdrawn from the first chamber. Because a greater volume of fluid isdisplaced in the first chamber than is taken into thesecond chamber inresponse to piston movement, only the fluidrequir'ed in the secondchamber is transmitted thereto by the pump. The

excess fluid is transferred through the third port and the third conduitto the source of hydraulic fluid, thereby compensating for actuatorunbalance.

To move the piston in the opposite direction, the pump withdraws fluidfrom the second chamber and supplies it through the first conduit to thefirst chamber. However, additional make-up fluid is required in that forany given piston movement, the volume of the first chamber increasesmore than the volume of the second chamber decreases. This make-up isprovided by the pump which draws the additional make-up fluid from thesource of the hydraulic fluid through the third conduit and supplies itvia the first conduit to the first chamber.

These desirable results are obtained by appropriately sizing andlocating the ports in the valve plate. The valve plate may be consideredas including first and second segments with a portion of the first portbeing in the first segment and with portions of the second and thirdports being in the second segment. The direction of fluid flow throughthe first segment is opposite the direction of fluid flow through thesecond segment. For example, during the intake stroke of the pump,hydraulic fluid may be drawn in from the first chamber through the firstport. During the discharge portion of the stroke, the fluid taken in isdischarged through the second and third ports, i.e., to the secondchamber and to the source of hydraulic fluid to thereby compensate forthe unbalance. The pump is preferably housed in a casing filled withhydraulic fluid and this may form a portion of the third conduit or aportion of the source of hydraulic fluid.

According to the second method of compensating for actuator unbalance,the present invention provides first and second conduits providingcommunication between the first and second chambers and the pump. Athird conduit connects the first conduit with a source of hydraulicfluid. The first and second conduits contain first and second valves,respectively, for controlling the flow of hydraulic fluid into and outof their associated chambers. A control valve selectively opens andcloses the third conduit.

According to this form of the invention, the first valve means and thecontrol valve are open when fluid is being withdrawn from the firstchamber by the pump. This permits excess fluid to flow through the thirdconduit means to the source of hydraulic fluid.

One feature of this invention is the provision of a restriction in thethird conduit to provide back pressure for the pump to prevent starvingof the pump. This restriction may be a simple, inexpensive, relief valvewhich is set at the desired pump back pressure.

Another feature of this form of the invention is that a common valveactuator can be utilized to open both the control valve and the firstvalve means when the first and third conduits are on the intake side ofthe pump. The common actuator is preferably responsive to the secondconduit being on the discharge sideofthe pump for opening the controlvalve and the first valve means-This represents a simplification of thestructure and a reduction in the cost of manufacture.

When the piston is moving in a-direction to enlarge the first chamber,make-up hydraulic fluid must be provided. This is preferably provided bya fourth conduit which interconnects the second conduit with the sourceof hydraulic fluid.

Another advantage of the present invention is that the first and secondvalve means automatically hold the piston in a fixed position in theabsence of any command for piston movement. Thus, the pump is notrequired to hold pressure in a stand-by condition. Another advantage ofthis construction is that in the event of a hydraulic failure resultingin loss of hydraulic fluid outside of the cylinder, the first and secondvalve means hold the piston in position.

The invention can best be understood by reference to the followingdescription taken in connection with the accompanying illustrativedrawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a diagrammatic view of oneform of hydraulic system constructed in accordance with the teachings ofthis invention.

FIG. 2 is a planned view of one form of valve plate which may beutilized by the pump which forms a portion of the system showing FIG. 1.

FIG. 3 is a diagrammatic view ofa second form of hydraulic systemconstructed in accordance with the teachings of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a hydraulic system11 constructed in accordance with the teachings of this invention.Generally the hydraulic system 1 1 includes a hydraulic actuator 13, apump 15, a motor 17 for driving the pump, and a motor controller 19.

The actuator 13 includes a housing or cylinder 21, a piston 23 slidablein the cylinder, and a connecting rod 25 for joining the piston to acontrollable member 26, the position of which is to be controlled. Thepiston 23 divides the cylinder 21 into chambers 27 and 29. Because ofthe presence of the rod 25 in the chamber 29, movement of the piston 23causes displacement of more hydraulic fluid in the chamber 27 than istaken into the chamber 29. Stated differently, the volume of the chamber27 changes a greater amount than the volume of the chamber 29 inresponse to predetermined displacement of the piston 23. Actuators ofthis type are known as unbalanced actuators.

The chambers 27 and 29 communicate with the exterior of the cylinder 21through ports 31 and 33, respectively. The flow of hydraulic fluid toand from the chambers 27 and 29 is controlled by identical,pilotoperated check valves 35 and 37. Of course, the specific form ofthe valves 35 and 37 can be varied by those having ordinary skill in theart.

The check valve 35 includes a hollow housing 39 defining a valve seat41. A valve element in the form of a ball 43 is urged by a spring 45into fluid tight sealing relationship with the valve seat 41.

The valve element 43 can be moved away from the valve seat 41 to openthe valve 35 by an actuator 47 which is slidable in the housing 39 andwhich is normally biased by a spring 48 out of engagement with the valveelement 43. The valve 37 is identical to the valve 35 and parts of thevalve 37 corresponding to parts of the valve 35 are designated bycorresponding reference numerals followed by the letter a.

Conduits 49 and 51 interconnect the pump and the valves 35 and 37,respectively. The conduits 49 and 51 are placed in communication with asource 53 of hydraulic fluid by conduits 55 and 57, respectively.

Check valves 59 and 61 in the conduits 55 and 57 allow for the flow ofhydraulic fluid out of the source 53 and prevent back flow to the sourcethrough the conduits 55 and 57. In the embodiment illustrated, thesource 53 is closed and pressurized.

The conduit 49 is placed in communication with the actuator 47a by theconduit 55 and another conduit 63.

' The actuator 47 is placed in communication with the conduit 51 by aconduit 65.

The pump 15 is a reversible, variable speed barreltype pump. Pumps ofthis type are known and disclosed, for example, in US. Pat. Nos.2,525,934 and 3,291,067. The pump 15 includes a cylinder barrel 67suitably mounted for rotation, a plurality of cylinders 69 in the barreland pistons 71 mounted for reciprocation in the cylinders 69,respectively. The cylinders communicate with ports 74, respectively. Thepistons 71 are driven by plungers 73 which rotate with the cylinder 69and which bear against a fixed cam plate 75. Accordingly, as the barrel67 rotates, the pistons 71 are reciprocated in their respectivecylinders 69. Reciprocation of the pistons 71 causes intake of fluidinto the cylinders 69 during one part of each cycle and discharge ofsuch fluid during another part of the cycle. This is standard barrelpump operation.

The pump 15 also includes a valve plate 77 which is stationary duringrotation of the barrel 67. As shown in FIG. 2, the valve plate 77 hasthree arcuate ports 79, 8l'and 83 separated by commutating lands 84. Thevalve plate 77 may be considered as being divided into segments 85 and87 one of which if for intake and the other of which is for discharge.In the embodiment illustrated, this division is made by an imaginaryline 89. The top of the line 89 represents top dead center, and thebottom of the line 89 represents bottom dead center. In the embodimentillustrated, the port 79 and relatively small portions of the ports 81and 83 are located in the segment 85. Relatively larger portions of theports 81 and 83 are located in the segment 87.

The ports 79, 81 and 83 are in communication with the conduits 49 and 51and with the interior of a casing 90 for the pump 15. A conduit 91provides communication between the interior of the casing 90 and thesource 53 of hydraulic fluid under pressure. With the barrel 67 of thepump 15 rotating in a first direction, the slot 79 and the portions ofthe slots 81 and 83 in the segment 85 are in communication with thecylinders 69 during the intake stroke and the portions of the slots 81and 83 in the segment 87 are incommunication with the cylinders 69during the discharge stroke.

The pump 15 can be driven in either direction and at different speeds bythe motor 17 which is a variable speed, reversible, electrical motor.The discharge rate of the pump 15 is a function of the speed at whichthe pump is driven with the discharge rate increasing as the rate atwhich the pump is driven increases.

The motor controller 17 can be any device suitable for varying the speedand direction of rotation of the motor 17. The controller 19 may beoperated automatically or manually. By way of example, the motorcontroller 19 may include the control system shown in common assigneescopending application, Ser. No. l69,345, filed on Aug. 5, I971, entitledAcoustic Control System, and naming Curtis E. Stevens as the inventor.Because motor speed is variable the cam plate 75 need not be adjustablymounted.

If it is desired to move the piston 23 to the left, the motor controller19 causes the motor 17 to rotate in a first direction. This places theconduits 49 and 51 on the intake and discharge, respectively, of thepump 15.

The motor 17 drives the barrel in a direction so that the ports and portportions on the segment 85 of the valve plate 77 are on the intake sideof the pump and the port portions on the segment 87 are on the dischargeside of the pump. Assuming that the barrel 67 is rotating in thedirection of the arrow A in FIG. 2, each piston 71 will draw fluid intoits associated cylinder 69 through the port 83 while that pistonconfronts the portion of the slot 83 on the segment 85. Similarly, fluidis drawn into each of the cylinders 69 through the slot 79 and theportion of the slot 81 on the segment 85 while the cylinders confrontthese slots in the segment 85. The quantity of fluid drawn into thecylinders from each of the ports 79, 81 and 83 is a function of thelengths and angular orientations of the respective ports on the segment85. In the embodiment shown in FIG. 2, a relatively large quantity offluid is drawn in from the conduit 49 through the slot 79 and minorquantities of fluid are drawn in from the case 90 through the slot 83and fromthe conduit 51 through the slot 81. Minor quantities are drawnin through the slots 81 and 83 because (I) their lengths on the segment85 are relatively short, and (2) they are located near bottom and topdead center, respectively, where the rate of pump piston movement isrelatively slow. Drawing in fluid through the slot 81 does not cause aloss of pressure in the conduit 51 because the pump 15 includes asufficient number of pistons, e.g., 7 or 9, so that at any instant morefluid is being discharged into the slot 81 and the conduit 51 than isbeing withdrawn.

With the pump 15 being driven in the direction of the arrow A in FIG. 2,each of the pistons 71 discharges fluid through the slots 81 and 83 involumetric amounts which is a function of the lengths and angularorientation of these slots. Because fluid is drawn into the cylinders 69from the slot 81 and also discharged from the cylinders 69 into the slot81, the effectiveness of the slot 81 for discharge purposes is reduced.Because the slot 79 is longer than the portion of the slot 81 on thesegment 87 and located where pumppiston velocity is greater, the pumpdraws in more fluid from the conduit 49 than it discharges into theconduit 51. This amount can be accurately apportioned so as to preciselycompensate for the unbalance of the actuator 13. The fluid which is notdischarged'into the conduit 51 through the port 81 is discharged throughthe slot 83, the case 90 and the conduit 91 to the source 53. It shouldbe understood that the valve plate 77 could be constructed in differentways so long as more fluid vis drawn in from the conduit 49 than isdischarged into the conduit 51.

Each of the ports 74 is sufficiently long so that the associatedcylinder 69 is always in communication with at least one of the slots79, 81 and 83. In FIG. 2 one of the ports 74 is illustrated between theslots 79 and 83 and the remaining ports 74 are not shown. In theposition of the port 74 shown in FIG. 2, it communicates with the slots79 and 83.

When the conduit 51 is on the discharge side of the pump 15, theactuator 47 is urged by the fluid under pressure in the conduit 65upwardly as viewed in FIG. 1 to move the valve element 43 off of theseat 41 against the biasing force of the spring 45. This permits thepump to draw fluid from the chamber 27 through the conduit 49.

The hydraulic fluid at discharge pressure in the conduit 51 acts tounseat the valve element 43a so that hydraulic fluid is supplied to thechamber 29 through the valve 37. The pump 15 can also draw fluid fromthe source 53 through the check valve 59 and the conduit 55. This may benecessary to develop enough pressure in the conduit 51 to unseat thevalve element 43. Drawing of fluid from the source 53 may also benecessary to make-up for leakage.

As the piston 23 moves to the left, it displaces a greater volume offluid in the chamber 27 than is taken into the chamber 29. However,because of the arrangement of the valve plate 77, the pump can beadapted to compensate precisely for this unbalance. Specifically, excesshydraulic fluid discharged from the chamber 27 is delivered through theport 83 to the case 90.

The rate at which the piston 23 moves can be controlled by the speed ofthe motor 17. Thus, for inching, the motor 17 may be caused by the motor19 to turn very slowly so that the piston 23 can be accuratelypositioned. When the desired position of the piston 23 has been reached,the motor 17 is de-energized and thereafter the normally closed checkvalves 35 and 37 maintain the piston at the preset position. If afailure causes loss of fluid pressure outside of the actuator 13, thenormally closed check valves 35 and 37 continue to maintain the piston23 in the preset position.

To move the piston 23 to the right as viewed in FIG. 1, the motor 17 isenergized by the controller 19 in the opposite to the direction A inFIG. 2. This causes the pump 15 to be driven in a direction opposite tothe direction in which it was initially driven. The result is that thesegment 87 of the valve plate 77 becomes theintake segment and thesegment becomes the discharge segment. Accordingly, as the barrel 67rotates, it draws in fluid through the portions of ports 81 and 83 onthe segment 87 and discharges the fluid through the port 79 and theportions of the ports 81 and 83 on the segment 85. Because of therelative lengths and orientations of the slots 79 and 81, the pumpdischarges more fluid into the conduit 49 through the slot 79 than itdraws in from the conduit 51 through the slot 81. The additional fluidis supplied from the source 53 through the conduit 91, the case 90 andthe slot ,83.

With the conduit 49 on the discharge side of the pump 15, fluid underpressure is transmitted through the conduit 63 to cause the actuator 47ato unseat the valve element 43a. This permits the pump 15 to draw fluidout of the chamber 49 and into the conduit 51.

The hydraulic fluid at discharge pressure in the conduit 49 actsdirectly against the valve element 43 to unseat the latter to therebypermit hydraulic fluid under pressure to be discharged into the chamber27 to move the piston 23 to the right as viewed in FIG. 1. If makeup isrequired for start-up or during operation, it can be obtained from thesource .53 through the check valve 61 and the conduit 57.

FIG. 3 shows a hydraulic system 111 constructed in accordance with theteachings of this invention. The hydraulic system 111 is very similar tothe hydraulic system 11 except that it provides alternate means forcompensating the unbalance of the actuator. The portions of thehydraulic system 1 11 corresponding to portions of the hydraulic system11 are designated by corresponding referenced characters preceded by thenumeral 1.

Generally, the hydraulic system 11 1 includes an actuator 113, a pump115, a motor 117, and a motor controller 119. The actuator 113, themotor 117, and the motor controller 1 19 are identical to thecorresponding elements described hereinabove with reference to FIG. 1.The pump 115 is preferably any reversible, variable speed, positivedisplacement pump and in the embodiment illustrated is a gear pump.

Pilot operated check valves 135 and 137 control the flow of hydraulicfluid into and out of the chambers 127 and 129, respectively. The checkvalve 137 is identical to the check valve 37 shown in FIG. 1. The checkvalve 135 is identical to the check valve 35 shown in FIG. 1 except thatthe actuator 147 includes a pair of lands defining a groove 210 and thebody of the valve includes ports 203 and 205. Thus, the actuator 147 andthe body of the valve 135 cooperate to form a control valve 206 which isnormally biased to the closed position shown in FIG. 3. However, it canbe moved to an open position in which the groove 201 providescommunication between the ports 203 and 205.

The pump 115 has two ports, 207 and 209, which are connected by conduits149 and 151, respectively, to the valves 135 and 137. A conduit 211provides communication between the conduit 149 and the source 153 ofhydraulic fluid under pressure. A restriction which, in the embodimentillustrated is in the form of a pressure relief valve 213, is positionedin the conduit 211 intermediate the control valve 206 and the source153.-

As in the embodiment of FIG. 1, conduits 165 and 157 providecommunication between the conduit 151 and the actuator 147 and thesource 153, respectively. Similarly, a conduit 155 providescommunication between the conduit 149 and the source 153 and the conduit163 provides communication between the conduit 149 and theactuator 147a.

In the operation of the system 111, the motor controller 119 energizesthe motor 117 to cause movement of the piston 123 in the desireddirection, such as to the left as viewed in FIG. 3. Motor speed 117 issimilarly adjusted so that the piston 123 will move at the desired rate.The pump 115 is driven by the motor 117. In order to move the piston 123to the left, the port 207 is the intake port and the port 209 is thedischarge port. With the conduit 151 on the discharge side of the pump115, the hydraulic fluid under pressure in the conduit 165 moves theactuator 147 upwardly to unseat the valve element 143 and to open thecontrol valve 206 by aligning the groove 201 with the ports 203 and 205.Thus, the control valve 206 and the valve 135 are opened by a singleactuator. The unseating of the valve element 143 allows the pump 115 todraw hydraulic fluid from the chamber 127.

The opening of the control valve 206 provides communication between theintake of the pump 115 and the relief valve 213. The conduit 211, thecontrol valve 206 and the pressure relief valve 213 provide means fordumping excess hydraulic fluid from the chamber 127. This is the excesshydraulic fluid which is not required in the chamber 129 due to theunbalance of the actuator 113.

The relief valve 213 is set at a preselected pressure which ispreferably relatively low to thereby provide back pressure for the pump.If the relief valve 213 or some other restriction were not present, theback pressure on the pump may be insufficient and cavitation may result.

The hydraulic fluid discharged by the pump unseats the valve element143a to permit this hydraulic fluid to be supplied to the chamber 129.This causes the piston 123 to move to the left at a rate which is afunction of the velocity at which the pump 115 is being driven. When thedesired position of the piston 123 has been reached, the motor 117 isde-energized by the controller 119 and thereafter the check valves and137 maintain the piston 123 in the preset position. Ordinarily, with thepiston 123 being moved to the left as viewed in FIG. 3, there will be anexcess of hydraulic fluid which is returned to the source 153 via theconduit 211, the control valve 206, and the pressure relief valve 213.However, if for any reason make-up hydraulic fluid is required, it canbe supplied from the source 153 through the check valve 161 and theconduit to the conduit 149.

To move the piston 123 to the right as viewed in FIG. 3, it is onlynecessary to reverse the direction of the motor 117, thereby reversingthe direction in which the pump 115 is driven. This causes the conduit149 to be on the discharge side of the pump 115 with the result that theactuator 147a is moved upwardly to unseat the valve element 143a. Thisallows the hydraulic fluid in the chamber 129 to escape into the conduit151 to feed the pump 115. In addition, the hydraulic fluid underpressure in the conduit 149 unseats the valve element 143 to permit thesupply of fluid under pressure to the chamber 127 and consequentmovement of the piston 123 to the right. As the conduit 15] is on theintake side of the pump 115, the control valve 206 remains in the closedposition shown in FIG. 3.

To move the piston 123 to the right, it is necessary to supply morehydraulic fluid to the chamber 127 than can be drawn from the chamber129. The additional fluid is supplied from the source 153 through thecheck valve 159 and the conduit 157 to the conduit 151. Of course, thecheck valves 159 and 161 prevent reverse flow of hydraulic fluid to thesource 153.

Although exemplary embodiments of the invention have been shown anddescribed, manychanges, modifications and substitutions may be made byone having ordinary skill in the art without necessarily departing fromthe spirit and scope of this invention.

We claim:

1. A hydraulic system comprising:

an actuator including a housing and a fluidresponsive member movable insaid housing, said member and said housing defining first and secondchambers on opposite sides, respectively, of said member;

a reversible, variable speed pump, the output of said pump being afunction of the speed at which said pump is driven;

conduit means for connecting said pump to both of said chambers wherebysaid pump can selectively pump hydraulic fluid from the first chamber tothe second chamber and from the second chamber to the first chamber tothereby move the fluidresponsive member;

reversible, variable speed drive means for driving said pump;

means for controlling the direction in which and the speed at which saidpump is driven by said drive means whereby the direction which the pumpis driven controls the direction of movement of the fluid-responsivemember and the speed at which the pump is driven controls the rate ofmovement of the fluid-responsive member;

said actuator being an unbalanced actuator whereby the change in volumeof the first chamber is greater than the change in volume of the secondchamber for a given displacement of the pressure-responsive member;

said conduit means including a first conduit leading from the pump tothe first chamber and a second conduit leading from the pump to thesecond chamber;

a third conduit leading from the pump to a source of hydraulic fluid;

said pump including means for supplying make-up from the third conduitthrough the first conduit to the first chamber when the volume of thefirst chamber is being increased and means for dumping excess hydraulicfluid from the first chamber through the first conduit tosaid sourcewhen the volume of the first chamber is being decreased; and

said pump including a barrel pump and a valve plate, said valve platehaving first, second and third ports therein communicating,respectively, with said first, second and third conduits.

2. A hydraulic system as defined in claim 1 wherein said drive meansincludes a reversible, variable speed electric motor, said pumpincluding a positive displacement pump.

3. A hydraulic system comprising:

a differential actuator including a housing and a piston movable in saidhousing, said piston dividing said housing into first and secondchambers on opposite sides of said piston, respectively, wherebymovement of said piston causes the volumes of said chambers to change;

said actuator including means for causing the volume of said firstchamber to undergo a greater change than the volume of said secondchamber in response to a predetermined amount of piston displacement;

a reversible pump having first and second ports;

a first conduit interconnecting the first port and said first chamber;

a second conduit connecting said second port and said second chamberwhereby the pump can pump hydraulic fluid through said conduits betweensaid chambers to thereby move the piston;

drive means for driving said pump, said drive means being reversible tochange the direction in which hydraulic fluid is pumped by said pump tothereby control the direction in which the; piston is moved;

first valve means operatively associated with the first conduit andresponsive to the first conduit being on the discharge side of the pumpto open to permit hydraulic fluid to be supplied to the first chamberthrough the first conduit;

second valve means operatively associated with the second conduit andresponsive to the second conduit being on the discharge side of the pumpto open to permit hydraulic fluid to be supplied to the second chamberthrough the second conduit;

a first valve actuator responsive to the second conduit being on thedischarge side of the pump for opening said first valve means to permithydraulic fluid to be drawn from the first chamber by said P p;

a third conduit leading from said first conduit for dumping excesshydraulic fluid when said first chamber is being reduced in volume bysaid movement of the piston;

a control valve for closing said third conduit when said first conduitis on the discharge side of the pump and for opening said third conduitwhen said second conduit is on the discharge side of the P p meansdefining a restriction in said third conduit to provide back pressurefor said pump when said control valve is open; and

means for supplying make-up hydraulic fluid to said first chamber whenthe volume of said first chamber is being increased.

4. A hydraulic system as defined in claim 3 wherein said restriction isdownstream of said control valve.

5. A hydraulic system as defined in claim 3 wherein said first valveactuator also opens said control valve whereby a single valve actuatoroperates both said first valve means and said control valve.

6. A hydraulic system as defined in claim 3 wherein said reversibledrive means is a variable speed reversible drive means, therebyproviding for better control of the movement of said piston.

7. A hydraulic system comprising:

a differential actuator including a housing and a piston movable in saidhousing, said piston dividing said housing into first and secondchambers on opposite sides of said piston, respectively, wherebymovement of said piston causes the volumes of said chambers to change;

said actuator including means for causing the volume of said firstchamber to undergo a greater change than the volume of said secondchamber in response to a predetermined amount of piston displacement;

a reversible pump having first and second ports;

a first conduit connecting the first port and said first chamber;

a second conduit connecting said second port and said second chamberwhereby the pump can pump hydraulic fluid through said conduits andbetween said chambers to thereby move the piston, the direction of fluidflow through said conduits being a function of the direction in whichsaid pump is driven;

reversible drive means for driving said pump to thereby control thedirection in which the piston is moved;

first valve means operatively associated with the first conduit andresponsive to the first conduit being on the discharge side of the pumpto open to permit hydraulic fluid to be supplied through the firstconduit to the first chamber;

a third conduit leading from said first conduit for dumping excesshydraulic fluid from the first chamber when said first chamber is beingreduced in volume by said movement of the piston;

a control valve for opening and closing said third conduit;

a valve actuator for opening said control valve and said first valvemeans in response to said second conduit being on the discharge side ofsaid pump whereby a single valve actuator opens said first valve meansto allow the pump to draw fluid from the first chamber and opens thecontrol valve to permit dumping of any excess hydraulic fluid throughthe third conduit; and

means for providing make-up fluid to the first chamber when the volumeof the first chamber is being increased.

8. A hydraulic system as defined in claim 7 wherein said reversibledrive means includes a reversible variable speed drive means wherebyfine control of piston movements can be accomplished.

9. A hydraulic system comprising:

an actuator including a housing and a piston movable in said housing,said actuator being an unbalanced actuator whereby movement of saidpiston causes a greater change in said piston volume on one side of thepiston than on the other side of the piston;

a reversible barrel-type pump, including a valve plate having first,second and third ports;

a first conduit leading from said first port to said one side of piston;

a second conduit leading from the second port to said other side of thepiston whereby said pump can pump hydraulic fluid from said one side ofthe piston to said other side of the piston and from said other side ofthe piston to said one side of the piston with the direction of fluidflow being a function of the direction in which the pump is driven;

a third conduit leading from the third port to a source of hydraulicfluid;

said valve plate having first and second segments, said first port beingat least partially on said first segment and said second and third portsbeing at least partially on said second segment;

said first 'and second segments being on the intake and discharge sides,respectively, of said pump when said pump is being driven in onedirection and said first and second segments being on the discharge andintake sides, respectively, of said pump whensaid pump is driven in theopposite direction, said third conduit drawing hydraulic fluid from saidsource of hydraulic fluid when said pump is being driven in saidopposite direction and supplying hydraulic fluid to said source when thepump is driven in said one direction; and

said ports being sized and positioned on said segments to substantiallycompensate for the unbalanee of said actuator whereby said pump suppliesmore hydraulic fluid to said one side of said piston than it draws fromsaid other side of said piston and supplies less hydraulic fluid to saidother side of said piston than it draws from said one side of saidpiston.

10. A hydraulic system as defined in claim 9 wherein a region of saidsecond port is on said first segment.

11. A hydraulic system as defined in claim 9 including a casing, saidpump and at least a portion of said source being within said casing.

12. A hydraulic system as defined in claim 9 including a reversible,variable speed drive means for driving said pump.

13. A hydraulic system comprising:

an actuator including a housing and a piston movable in said housing,said actuator being an unbalanced actuator whereby movement of thepiston causes a greater change in volume on one side of the piston thanon the other side of the piston;

a pump including first, second and third ports;

a first conduit for providing communication between said first port andsaid one side of said piston;

a second conduit leading from the second port to said other side of thepiston;

a third conduit for providing communication between said third port anda source of hydraulic fluid;

said pump being drivable in one direction to pump hydraulic fluid fromsaid one side of said piston through said first and second conduits tosaid other side of said piston and being drivable in the oppositedirection to pump hydraulic fluid from said other side of the pistonthrough said second and first conduits to said one side of said piston;and

said pump including first means for directing fluid from said firstconduit to said second and third conduits when said first conduit is onthe intake side of said pump and second means for directing fluid fromthe second and third conduits to the first conduit when said firstconduit is on the discharge side of said pump to thereby at leastpartially compensate for the unbalance of the actuator.

14. A hydraulic system as defined in claim 13 wherein said pump includesa barrel-type pump and said first and second means include a valve platehaving a plurality of ports therein sized and positioned tosubstantially compensate for the unbalance of the actua-

1. A hydraulic system comprising: an actuator including a housing and afluid-responsive member movable in said housing, said member and saidhousing defining first and second chambers on opposite sides,respectively, of said member; a reversible, variable speed pump, theoutput of said pump being a function of the speed at which said pump isdriven; conduit means for connecting said pump to both of said chamberswhereby said pump can selectively pump hydraulic fluid from the firstchamber to the second chamber and from the second chamber to the firstchamber to thereby move the fluid-responsive member; reversible,variable speed drive means for driving said pump; means for controllingthe direction in which and the speed at which said pump is driven bysaid drive means whereby the direction which the pump is driven controlsthe direction of movement of the fluid-responsive member and the speedat which the pump is driven controls the rate of movement of thefluidresponsive member; said actuator being an unbalanced actuatorwhereby the change in volume of the first chamber is greater than thechange in volume of the second chamber for a given displacement of thepressure-responsive member; said conduit means including a first conduitleading from the pump to the first chamber and a second conduit leadingfrom the pump to the second chamber; a third conduit leading from thepump to a source of hydraulic fluid; said pump including means forsupplying make-up from the third conduit through the first conduit tothe first chamber when the volume of the first chamber is beingincreased and means for dumping excess hydraulic fluid from the firstchamber through the first conduit to said source when the volume of thefirst chamber is being decreased; and said pump including a barrel pumpand a valve plate, said valve plate having first, second and third portstherein communicating, respectively, with said first, second and thirdconduits.
 2. A hydraulic system as defined in claim 1 wherein said drivemeans includes a reversible, variable speed electric motor, said pumpincluding a positive displacement pump.
 3. A hydraulic systemcomprising: a differential actuator including a housing and a pistonmovable in said housing, said piston dividing said housing into firstand second chambers on opposite sides of said piston, respectively,whereby movement of said piston causes the volumes of said chambers tochange; said actuator including means for causing the volume of saidfirst chamber to undergo a greater change than the volume of said secondchamber in response to a predetermined amount of piston displacement; areversible pump having first and second ports; a first conduitinterconnecting the first port and said first chamber; a second conduitconnecting said second port and said second chamber whereby the pump canpump hydraulic fluid through said conduits between said chambers tothereby move the piston; drive means for driving said pump, said drivemeans being reversible to change the direction in which hydraulic fluidis pumped by said pump to thereby control the direction in which thepiston is moved; first valve means operatively associated with the firstconduit and responsive to the first conduit being on the discharge sideof the pump to open to permit hydraulic fluid to be supplied to thefirst chamber through the first conduit; second valve means operativelyassociated with the second conduit and responsive to the second conduitbeing on the discharge side of the pump to open to permit hydraulicfluid to be supplied to the second chamber through the second conduit; afirst valve actuator responsive to the second conduit being on thedischarge side of the pump for opening said first valve means to permithydraulic fluid to be drawn from the first chamber by said pump; a thirdconduit leading from said first conduit for dumping excess hydraulicfluid when said first chamber is being reduced in volume by saidmovement of the piston; a control valve for closing said third conduitwhen said first conduit is on the discharge side of the pump and foropening said third conduit when said second conduit is on the dischargeside of the pump; means defining a restriction in said third conduit toprovide back pressure for said pump when said control valve is open; andMeans for supplying make-up hydraulic fluid to said first chamber whenthe volume of said first chamber is being increased.
 4. A hydraulicsystem as defined in claim 3 wherein said restriction is downstream ofsaid control valve.
 5. A hydraulic system as defined in claim 3 whereinsaid first valve actuator also opens said control valve whereby a singlevalve actuator operates both said first valve means and said controlvalve.
 6. A hydraulic system as defined in claim 3 wherein saidreversible drive means is a variable speed reversible drive means,thereby providing for better control of the movement of said piston. 7.A hydraulic system comprising: a differential actuator including ahousing and a piston movable in said housing, said piston dividing saidhousing into first and second chambers on opposite sides of said piston,respectively, whereby movement of said piston causes the volumes of saidchambers to change; said actuator including means for causing the volumeof said first chamber to undergo a greater change than the volume ofsaid second chamber in response to a predetermined amount of pistondisplacement; a reversible pump having first and second ports; a firstconduit connecting the first port and said first chamber; a secondconduit connecting said second port and said second chamber whereby thepump can pump hydraulic fluid through said conduits and between saidchambers to thereby move the piston, the direction of fluid flow throughsaid conduits being a function of the direction in which said pump isdriven; reversible drive means for driving said pump to thereby controlthe direction in which the piston is moved; first valve meansoperatively associated with the first conduit and responsive to thefirst conduit being on the discharge side of the pump to open to permithydraulic fluid to be supplied through the first conduit to the firstchamber; a third conduit leading from said first conduit for dumpingexcess hydraulic fluid from the first chamber when said first chamber isbeing reduced in volume by said movement of the piston; a control valvefor opening and closing said third conduit; a valve actuator for openingsaid control valve and said first valve means in response to said secondconduit being on the discharge side of said pump whereby a single valveactuator opens said first valve means to allow the pump to draw fluidfrom the first chamber and opens the control valve to permit dumping ofany excess hydraulic fluid through the third conduit; and means forproviding make-up fluid to the first chamber when the volume of thefirst chamber is being increased.
 8. A hydraulic system as defined inclaim 7 wherein said reversible drive means includes a reversiblevariable speed drive means whereby fine control of piston movements canbe accomplished.
 9. A hydraulic system comprising: an actuator includinga housing and a piston movable in said housing, said actuator being anunbalanced actuator whereby movement of said piston causes a greaterchange in said piston volume on one side of the piston than on the otherside of the piston; a reversible barrel-type pump, including a valveplate having first, second and third ports; a first conduit leading fromsaid first port to said one side of piston; a second conduit leadingfrom the second port to said other side of the piston whereby said pumpcan pump hydraulic fluid from said one side of the piston to said otherside of the piston and from said other side of the piston to said oneside of the piston with the direction of fluid flow being a function ofthe direction in which the pump is driven; a third conduit leading fromthe third port to a source of hydraulic fluid; said valve plate havingfirst and second segments, said first port being at least partially onsaid first segment and said second and third ports being at leastpartially on said second segment; said first and second seGments beingon the intake and discharge sides, respectively, of said pump when saidpump is being driven in one direction and said first and second segmentsbeing on the discharge and intake sides, respectively, of said pump whensaid pump is driven in the opposite direction, said third conduitdrawing hydraulic fluid from said source of hydraulic fluid when saidpump is being driven in said opposite direction and supplying hydraulicfluid to said source when the pump is driven in said one direction; andsaid ports being sized and positioned on said segments to substantiallycompensate for the unbalance of said actuator whereby said pump suppliesmore hydraulic fluid to said one side of said piston than it draws fromsaid other side of said piston and supplies less hydraulic fluid to saidother side of said piston than it draws from said one side of saidpiston.
 10. A hydraulic system as defined in claim 9 wherein a region ofsaid second port is on said first segment.
 11. A hydraulic system asdefined in claim 9 including a casing, said pump and at least a portionof said source being within said casing.
 12. A hydraulic system asdefined in claim 9 including a reversible, variable speed drive meansfor driving said pump.
 13. A hydraulic system comprising: an actuatorincluding a housing and a piston movable in said housing, said actuatorbeing an unbalanced actuator whereby movement of the piston causes agreater change in volume on one side of the piston than on the otherside of the piston; a pump including first, second and third ports; afirst conduit for providing communication between said first port andsaid one side of said piston; a second conduit leading from the secondport to said other side of the piston; a third conduit for providingcommunication between said third port and a source of hydraulic fluid;said pump being drivable in one direction to pump hydraulic fluid fromsaid one side of said piston through said first and second conduits tosaid other side of said piston and being drivable in the oppositedirection to pump hydraulic fluid from said other side of the pistonthrough said second and first conduits to said one side of said piston;and said pump including first means for directing fluid from said firstconduit to said second and third conduits when said first conduit is onthe intake side of said pump and second means for directing fluid fromthe second and third conduits to the first conduit when said firstconduit is on the discharge side of said pump to thereby at leastpartially compensate for the unbalance of the actuator.
 14. A hydraulicsystem as defined in claim 13 wherein said pump includes a barrel-typepump and said first and second means include a valve plate having aplurality of ports therein sized and positioned to substantiallycompensate for the unbalance of the actuator.